The present invention generally relates to a method of treating a separator for use in making bonded multi-layer, flat-plate electrochemical cell devices, such as rechargeable batteries and supercapacitors. More specifically, the invention describes a method of treating a separator for use in establishing persistent interfacial bonding between laminated planar electrode and the separator utilized in such electrochemical devices wherein the bonding is acieved at a low-temperature.
Widely deployed primary and secondary, rechargeable lithium-ion battery cells are typical of electrochemical devices to which the present invention is directed. Such cells comprise layers, or membranes, of respective positive and negative electrode compositions assembled with a coextensive interposed layer, or membrane, of electrically-insulating, ion-transmissive separator material. This multi-layer battery cell structure is normally packaged with a mobile-ion electrolyte composition, usually in fluid state and situated in part in the separator membrane, in order to ensure essential ionic conductivity between the electrode membranes during charge and discharge cycles of the battery cell.
One type of separator for this purpose is a microporous polyolefin membrane, either of single- or multi-layer structure, described, for example, in U.S. Pat. Nos. 5,565,281 and 5,667,911. When employed as rechargeable battery cell separators, these porous membranes not only effectively retain within their porous structure the essential fluid cell electrolyte compositions, but they also provide an additional advantage in that they possess an automatic cell xe2x80x9cshut-downxe2x80x9d feature that prevents uncontrolled heat buildup within the battery cell which might otherwise result, for instance during excessive cell recharging, in a dangerous explosive condition. This built-in safety mechanism occurs because the melting point range of the polyolefins utilized in the fabrication of the separator membranes is at the lower end of the danger zone of battery cell heat buildup. Thus, in the event of a run-away cell heating episode, the porous polyolefin separator membrane becomes heated to a point of melting and its pore structure collapses, thereby interrupting the essential ionic conductivity within the cell and terminating the electrochemical reaction before a dangerous condition ensues.
The packaging of battery cell structures has heretofore regularly taken the form of a metal xe2x80x9ccanxe2x80x9d, whether, for example, in elongated tubular or flattened prismatic shape, which has commonly been relied upon to not only contain the electrolyte component, but also to impart the significant stack pressure required to maintain close physical contact between the individual cell electrodes and the interposed separator member. This intimate contact, along with the composition of the electrolyte, is, as previously noted, essential to efficient ion transmission between electrodes during operation of the battery cell.
More recently, however, the profusion and continued miniaturization of electronic devices powered by Li-ion batteries and similar energy storage cells has generated a demand for a greater number of cell package shapes and dimensions, e.g., relatively broad, yet thin, lightweight packages having a significant degree of flexibility. For example, numerous end-use applications make thin, flexible envelope-style packages of polymer film more desirable than the prior rigid-walled high-pressure can containers. However, these more flexible packages are decreasingly capable of achieving and maintaining the substantial physical pressures required to ensure the noted essential intimate inter-layer contact throughout the battery cell.
In order to minimize the deleterious effect of degraded physical stack pressure previously relied upon to establish the necessary contact between cell layers, developers have progressed to the use of direct laminated adhesive bonding between electrode and separator layers to ensure their essential intimate contact. Typical of such innovations are battery cells utilizing polymer-based layer members, such as described in U.S. Pat. Nos. 5,456,000 and 5,460,904. In those fabrications, polymer compositions, preferably of poly(vinylidene fluoride) copolymers, which are compatible with efficient fluid electrolyte compositions are utilized in the physical matrix of both the electrode and the separator members to not only promote essential ionic conductivity, but also to provide a common composition component in those cell members which promotes strong interfacial adhesion between them within a reasonably low laminating temperature range. Such laminated, multilayer polymeric battery cells operate effectively with stable, high-capacity performance even though packaged in flexible, lightweight polymeric film enclosures.
Although such laminated battery cells, and like energy storage devices, have significantly advanced the art in miniaturized applications, the use of substantially non-porous polymeric matrices and membranes in their fabrication has deprived these devices of the desirable shut-down feature achieved when using the microporous polyolefin separator membranes. However, the high surface energy exhibited by the polyolefin membranes renders them highly abherent in nature and thus prevents their strong, permanent adhesion to electrode layer compositions, particularly within a reasonable temperature range which does not lead to melting or thermal collapse of the porous structure of the polyolefin membranes.
Some attempts have been made by electrochemical cell fabricators to combine, by simple solution overcoating or extrusion, the shut-down properties of porous separator membranes with the laminate adhesive properties of polymer compositions, for example, as described in U.S. Pat. Nos. 5,837,015 and 5,853,916. However, it has generally been found that the overcoating compositions significantly occlude or otherwise interfere with the porous structure of the polyolefin membranes and cause a deleterious decrease in electrolyte mobility and ionic conductivity. Further, the addition of substantial amounts of overcoating materials, increases the proportion of non-reactive components in a cell, thereby detracting from the specific capacity of any resulting energy storage device.
As an alternative approach to enabling the incorporation of microporous separator membranes into a laminated electrochemical cell structure, an attempt to modify the surface of the polyolefin membrane by application of a minimal layer of polymer composition has been made.
The polymer composition would not be of such excessive thickness as to occlude the porosity of the membrane, but rather would provide an intermediate transition in compatibility to the matrix polymer of preferred electrode cell layer compositions. Thus, for example, a thin layer from a dilute solution of poly(vinylidene fluoride) copolymer is applied to the microporous separator membrane when the membrane is intended to be employed in the fabrication of a battery cell by thermal lamination with electrodes comprising active compositions of a similar polymer. This modification has proven to be insufficient in itself to enable satisfactory interfacial bonding between cell component layers at lamination temperatures below the critical level which results in collapse of separator porosity and its attendant loss of effective ionic conductivity and desirable shut-down capability.
Therefore, there remains a need in the art to provide improved surface-modified microporous separator membranes for use in high-capacity, shut-down protected laminated electrochemical cells.
The present invention provides a method of treating surface-modified microporous separator membranes so that such membranes can be effectively used in electrochemical cells.
More particularly, the present invention comprises a method for facilitating the lamination of electrochemical cells at laminating temperatures which effect firm interfacial bonding between electrode and separator layers, yet are sufficiently low to avoid thermal collapse or other occlusion of the porous structure of the separator membranes, through the use of surface-modified microporous polyolefin separator membranes which have been treated in accordance with the present invention. The method of the present invention helps prevent loss of essential ionic conductivity and maintains thermal shut-down capability.
In general, the method of the present invention comprises initially applying to a surface-modified separator membrane a dilute solution of a primary plasticizer for the surface-modifying, polymeric membrane coating in a volatile organic solvent, and removing the volatile solvent, such as by evaporation in air, to deposit the plasticizer in the pores of the separator. The cell is further processed by applying an electrode to each surface of the surface-modified separator membrane; applying a moderate amount of heat and pressure to the multi-layer assembly to affect bonding; and removing any residual plasticizer from the assembly by heat and/or reduced pressure.
The treatment solution is preferably made up of about 10% to 30% of the plasticizer, and more preferably about 15% to 20% plasticizer. Useful plasticizers are moderately volatile and include alkylene carbonates, dialkyl phthalates, dialkyl succinates, dialkyl adipates, dialkyl sebacates, trialkyl phosphates, polyalkylene glycol ethers and mixtures thereof. The organic solvent is selected to be significantly more volatile than the plasticizer and to exhibit limited solvency toward the surface-modifying polymer of the separator membrane. Lower alcohols, ketones, esters, aliphatic hydrocarbons, halogenated solvents, chlorinated hydrocarbons, chlorinated fluorocarbons, and mixtures thereof are all useful. A sufficient amount of the plasticizer solution is applied to the membrane to ensure some significant intake of the solution within the pores of the membrane. The treatment solution may be applied by any appropriate method, such as coating, immersion or spraying.
Electrode membranes may be in the form of highly densified polymeric electrodes deposited on metal-foil current collectors, such as those used in liquid-electrolyte Li-ion cells, and/or densified and non-extracted and/or extracted plastic Li-ion electrodes such as those disclosed in U.S. Pat. Nos. 5,418,091; 5,429,891; 5,456,000; 5,460,904; 5,540,741; 5,571,634; 5,587,253; 5,607,485; wherein preferably at least one electrode has a reticulated metal current collector in the form of an expanded-metal grid, mesh, metallic non-woven material, etched foil or perforated foil.
Following application of the plasticizer/solvent solution, the volatile solvent is removed, such as by evaporation, which results in the deposition of the plasticizer superficially on the surface and in the pores of the separator membrane. The coated separator membrane is thereafter assembled in the usual manner between positive and negative electrode layers or membranes and the assemblage is laminated, e.g., between heated pressure rollers, at a temperature and pressure which does not significantly effect the porous structure; i.e. a temperature below the shutdown temperature, of the separator membrane. For example, lamination may be carried out between 70xc2x0 C. and 120xc2x0 C., and preferably between 90xc2x0 C. and 110xc2x0 C., and more preferably at about 100xc2x0 C., and with a linear load between 10 and 40 pounds per linear inch (lb/in) and more preferably between 20 and 30 lb/in. Advantageously, when processed in these temperature and pressure ranges, the deposited plasticizer now resident in and about the porous separator membrane exhibits its solvency toward and softens the surface-modifying polymer of the separator membrane, as well as the contiguous surface of the compatible electrode matrix polymer, and a close adhesive/ cohesive bond is formed between the electrode and separator membrane interfaces.
A minor amount of plasticizer insufficient to disrupt the modifying polymer layer may reside on the surface of the membrane at the outset of the lamination operation, however, a greater amount is forced from the pores of the separator membrane under the pressure of lamination and provides sufficient softening of the polymer interfaces to effect a deep intermingling of the surface polymers of the electrode and separator membranes. Subsequent to the lamination, and influenced by the slowly dissipating heat of the laminating operation, the remaining plasticizer volatilizes to promote a strong, unsoftened polymer bond at the electrode and separator membrane interfaces.
In and alternative embodiment of the present invention, the moderately volatile primary plasticizer is included in the electrode polymer matrix composition and is available from that source at the electrode and separator membrane interface to act upon the polymer layer of the separator membrane during the laminating operation.